1. Field of the Invention
The present invention relates to a piezoelectric-element drive device that can change a drive frequency, an electronic apparatus having a fluid pump using a piezoelectric element, and a method for controlling the driving frequency of a piezoelectric element.
2. Description of the Related Art
Piezoelectric-element drive circuits that are preferably used for liquid sprayers and various types of actuator have been proposed (e.g., refer to Japanese Patent Nos. 2618685 and 3115618.
In such a drive circuit, a transistor for switching is connected to a primary winding of a transformer and a piezoelectric element is connected to a secondary winding thereof. The drive circuit generates and controls a drive voltage for the piezoelectric element by performing a switching operation of the transistor.
As a drive circuit for a supersonic motor using a piezoelectric element, a drive circuit having a resonant frequency that resonates in parallel with the piezoelectric element is in order to generate a sinusoidal drive voltage at the secondary side even when switching using a transistor or the like is performed at the primary side has been proposed (e.g., refer to Japanese Patent No. 2976489.
Japanese Patent Nos. 2618685, 3115618, and 2976489 disclose technologies in which a resonant circuit including a piezoelectric element is formed at the secondary side of the transformer and the piezoelectric element is driven in the vicinity of the resonant frequency of the resonant circuit.
The driving in the vicinity of the resonant frequency is preferable because of high efficiency and low-power consumption.
The piezoelectric element (piezoelectric vibrator), however, has a resonant point that varies depending on a temperature or the like, for example, as disclosed in Japanese Patent No. 2618685, and thus typically involves adjusting the resonant point in accordance with variations in the temperature or the like.
Accordingly, in Japanese Patent Nos. 2618685 and 3115618, phase comparison of a voltage and a current at the secondary side of the transformer is performed to control a drive frequency corresponding to changes in the resonant point.